Engine can't make the RPM is used to, give a bit more info, is it just like it's hitting the wall (like when you hit 240km/h and can't push past the wind resistance) or is it misfiring at that point etc.???? It hasn't developed a massive exhaust leak or something has it?
If the WHOLE car is vibrating it could indicate a tailshaft problem (loose bolts, very worn out uni-joint, bowed/unbalanced shaft). If you just feel it though the steeering wheel it is more indicative of an unbabalanced wheel, perhaps a wheel balance weight fell off or the pot hole took a chuck of rubber off your tyre? A very unbalanced wheel could be felt through the whole car too.
The tail shaft wouldn't limit engine rpm, even a completely rooted tailshaft that is bowed will rev to the point where the car rattles apart and the tail shaft destructs.
It does seem strange to develop two completely independent problems at once, but in old cars it does happen.